The present invention relates to borehole logging instruments, and more particularly to the use of a borehole televiewer ("BHTV") as a dipmeter. Such a televiewer is described in U.S. Pat. No. 3,369,626, where the use thereof as a dipmeter is also suggested. The term "dipmeter" is used to refer to instruments that measure the dip angle of a bedding or fracture plane and the azimuth of the plane. Normally, the angle between the bedding or fracture plane and horizontal is referred to as the dip (or dip angle) of the plane, and the dip azimuth is measured with respect to geographic north by a line (sometimes called the "strike" of the plane) which is the line of intersection of a horizontal plane and the bedding or fracture plane, and is normal to the dip.
Conventionally, the dip and dip azimuth of the plane have been determined by a four arm electrical logging device that measures the resistivity of the various formations through which it passes. The resistivity is determined by each of the individual arms and separately recorded together with the orientation of one of the arms with respect to geographic or magnetic north. With this information and knowing the deviation or inclination of the borehole at the depth of interest and the azimuth of the deviation, one can calculate the dip and azimuth of the bedding or fracture plane. While this type of dipmeter has been conventionally used for many years, it cannot generally operate in boreholes filled with oil-based mud. Of course, if it is possible to replace the oil-based mud with a water-based mud without damaging the formation, then one can usually obtain electrical logging information.
Conventional dipmeter instruments also fail in those formations where the resistivity contrasts between the formations on one side of the bedding or fracture plane and the formations on the other side are not great enough to produce appreciable differences in the resistivity as measured by the instrument.
A need therefore remains for an improved method for logging the true dip angle and azimuth of earth formations using a borehole televiewer. A particular need remains for a method for logging the dip and dip azimuth of such formations in boreholes which are filled with an oil-based mud. Such a method should be sensitive, accurate, and should readily compensate for the adverse effects of borehole deviation and the dip inclination of the earth's magnetic field.